1. Field of the Invention
The present invention relates generally to an implantable cardiac stimulating apparatus. More particularly, the present invention relates to a rate adaptive cardiac rhythm management device having a first sensor for measuring a physiologic parameter reflecting metabolic demand and a second sensor for measuring a parameter reflecting the physical motion or activity of the patient, wherein the second sensor is used to generate a dynamic target pacing rate which the first sensor is optimized to over time, thereby reducing the time constant for the adaptation of the first sensor and minimizing the amount of clinical time required to initialize the cardiac rhythm management device.
2. Discussion of the Prior Art
A cardiac rhythm management device, or cardiac pacemaker, may be generalized as an implantable instrument designed to supplant some or all of an abnormal heart's natural pacing functions. Early attempts at cardiac rhythm management devices involved fixed-rate pacing where electrical stimulation signals are delivered to myocardial tissue at fixed intervals in order to ensure proper heart function. Although generally effective, this technique is nonetheless disadvantageous in that it does not allow the cardiac pacemaker to vary the rate of the electrical stimulation signals in response to variations in metabolic activity, such as during periods of rest or exercise, thereby resulting in a heart rate which is either too fast or slow relative to the metabolic needs of the patient. To overcome these deficiencies, a whole class of "rateadaptive" cardiac pacemakers have been developed that operate to sense some parameter correlating with metabolic need and then, using the sensed signal, derive a rate-controlling index for adjusting the pacing rate between a minimum or lower rate limit and a maximum or upper rate limit.
Physiologic parameters that are most commonly employed in rate-adaptive pacing may include any number of parameters indicative of metabolic demand, such as blood Ph, blood temperature, QT interval, pre-ejection interval, stroke volume, blood oxygen saturation, respiratory rate, and minute ventilation. A problem exists, however, in that these physiologic parameters respond relatively slowly to changes in the patient's level of exercise and, thus, can cause to patient to experience a hemodynamic deficiency due to the lag time between the onset of a new level of exercise and the reaction thereto by the pacemaker. In an effort to achieve a more rapid response to metabolic demand, various cardiac rhythm management devices have been developed which include a second sensing element for detecting the physical activity of the patient under the theory that motion or physical activity is directly correlative to the sinus rate of the patient and will thus allow the cardiac pacemaker to be more responsive to true metabolic demand.
U.S. Pat. No. 4,782,836 to Alt discloses one such dual-sensor rate-adaptive pacemaker, employing an activity sensor in conjunction with a blood temperature sensor wherein one of two algorithms is employed for relating blood temperature to pacing rate depending upon the level of patient exercise as detected by the activity sensor. U.S. Pat. No. 4,860,751 to Callaghan teaches yet another dual-sensor rate-adaptive pacemaker characterized in that the output of the activity sensor is utilized by control circuitry to enable the physiological sensor to monitor a selected physiologic parameter only if the physical activity of the patient exceeds a predetermined threshold. U.S. Pat. No. 4,905,697 to Heggs discloses the use of a blood temperature sensor for increasing the pacing rate of a pacemaker due to exercise and a motion sensor to trigger a decrease in the pacing rate following cessation of exercise. U.S. Pat. No. 4,926,863 to Alt teaches the use of an accelerometer for detecting the activity of the patient and a blood temperature sensor for detecting metabolic demand, wherein the accelerometer converts mechanical movement of the patient to a corresponding electrical signal which is combined with the sensed blood temperature parameter signal to confirm the metabolic state of the patient. U.S. Pat. No. 5,101,824 to Lekholm discloses a rate-adaptive pacemaker having two or more sensors for sensing physiologic parameters and patient activity, wherein an addressable rate matrix is employed to produce a specific pacing rate unique to each combination of sensor inputs.
The foregoing prior art references, however, all suffer a significant drawback in terms of the initialization process performed following the implantation of the cardiac pacemaker. The process of initialization involves having the physician set or program the cardiac pacemaker such that the sensors are appropriately tuned or optimized to allow the cardiac pacemaker to accurately respond to changes in the patient's metabolic demand. The algorithms used to control physiologic sensors typically have relatively long time constants of up to 30 minutes or more. The lengthy time constant is disadvantageous, however, in that it consumes a substantial amount of clinical time in order to effectuate the initialization. As will be appreciated, this is also costly and disadvantageous in that it effectively limits the number of patients whose pacemakers may be initialized within a given period of time so as to negatively impact the efficiency of the clinical operations. Invariably, physicians end up bypassing the lengthy automatic initialization process by manually setting the response slope of the physiologic sensor in an effort to send the patients home shortly after a post-implantation follow-up examination. Manual optimization of the sensors is disadvantageous in that it is typically based on "best guess" approximation which, of course, is highly subjective and more likely to result in non-optimal sensor rate settings.
Still further drawbacks exist with regard to the algorithms employed to adapt the physiologic and activity sensors. These algorithms, typically referred to as "automatic slope algorithms," are used to adapt a sensor response based on a feedback mechanism. One common feedback mechanism is dependent upon whether the pacing rate achieves a maximum sensor rate (MSR) within a predetermined time period. MSR is defined as the maximum pacing rate allowed as a result of sensor control and is typically programmed from 50-185 pulses per minute (ppm) in 5-ppm increments. Another common feedback mechanism is dependent upon whether the pacing rate achieves a sensor rate target (SRT) which is lower than the MSR within a predetermined time period. Algorithms of the first type are known to result in inappropriate response optimization in that it assumes that the patient exercises up to the programmed MSR in every time period. Algorithms of the second type require programming of a patient individual TSR which can be described as the typical maximum daily achieved rate. However, this is an arbitrary procedure since the physician will typically rely on subjective patient data to program this rate. Furthermore, both types of algorithms have very long time constants for optimization typically measured in months. This is disadvantageous, once again, in that it is contrary to the physician's goal of sending the patient home with an optimized response. Another disadvantage with these algorithms is that they typically result in extremely aggressive sensor response after a period of sedentary behavior or immobility.
A need therefore exists for an improved cardiac pacemaker with automatic response optimization of a physiologic sensor based on an activity sensor which overcomes the aforementioned deficiencies in the prior art.